Put the 700 g potatoes, unpeeled, in a 4-quart / 3.8 L pot with cold water to cover and 1 tbsp coarse salt. Bring to a boil, then simmer for 30 to 35 minutes, until a knife slides to the center with no resistance. Drain and let them sit for 15 minutes so the steam escapes.

Peel the potatoes while still warm, holding them with a kitchen towel, and rice them into a large bowl. Add the flour, egg, grated Casera, 1 tsp fine salt, and nutmeg. Mix with your hands for about 1 minute, until no dry flour shows and the dough is soft but not sticky. Stop there.

Line a 33 x 46 cm / 13 x 18 inch tray with parchment. Divide the dough into 8 portions of about 120 g. Press a Casera cube into the center of each, close the dough around it, roll into a ball, and flatten to a round about 7 cm wide and 3 cm thick. Chill for 10 minutes.

Heat 1 tbsp butter and 1 tbsp olive oil in a 30 cm / 12-inch nonstick or cast-iron skillet over medium heat until the butter foams. Sear 4 rounds for 4 to 5 minutes per side, without moving them, until the crust is deep golden and the centers are soft. Move to a plate and tent loosely with foil. Wipe the pan and repeat with the remaining butter, oil, and rounds.

While the second batch sears, melt 40 g butter with the sage leaves in an 18 cm / 7-inch saucepan over medium-low heat for 2 to 3 minutes, until the sage crisps and the butter smells nutty. Plate 2 rounds per person, spoon over the sage butter, dust with Grana Padano, and serve at once.